All depends what other evidence there is. Was he under the influence of any substances, was he so angry he wasn’t thinking straight, or maybe mentally unbalanced due to childhood trauma. People shoot other people all the time in a fit of anger or just because they are crazy, in all times of the day, even in front of witnesses and the charges stick.
I wonder why he headed down there with a loaded gun at all when he felt she and her dog were dangerous and there were weapons. Most people would not do that, maybe call or send a text instead. He did say at one point on a call to 911 something about “this is war.” Seem he came loaded to battle. He is lucky they lived and he was not shot.
The Current Status of GSR Examinations | FBI: Law Enforcement Bulletin
Gunshot residue examinations have continued to improve through research, advancements, and communication.